The present invention relates generally to marine jet propulsion units, and specifically to systems for mounting relatively higher-powered, marine jet propulsion into multi-passenger watercraft, particularly where the propulsion units employ indirect drive systems.
Conventional marine jet propulsion units are designed to be used instead of propeller-driven outboard or inboard marine motors. Some of the more significant advantages of jet propulsion units include the lack of a depending gear case, which allows the craft to have minimum contact with the water surface at high speed. This feature of jet propulsion units enables the operator to make tight turns while maintaining the boat in a generally horizontal orientation. Another feature of marine jet propulsion units is that the lack of a depending propeller enables the craft to be operated in shallower water without fouling.
A significant design criterion for mounting such jet propulsion units to watercraft hulls is the need to secure the unit to the hull in a manner which minimizes the transmission of power unit and/or gear train vibrations through the hull. Another design factor relates to the need to easily mount the power unit into the hull, which is an operation normally performed by the boat builder in a location remote from the point of manufacture of the propulsion unit.
One conventional jet propulsion unit is mounted to a hull having a mounting platform elevated from the base of the hull to define a channel. A generally rectangular mounting opening is defined in the mounting platform. The power head is mounted to a drive housing cover disposed above both the mounting platform and the channel. The drive housing cover thus overlaps the mounting opening in the mounting platform. A drive housing is secured to an underside of the drive housing cover, and is located below the platform and within the channel. A main elastomeric grommet circumscribes the drive housing at the platform level and is sandwiched between the drive housing cover and the platform primarily to prevent the entry of ambient water into the power head area. Another function of the grommet is to isolate power head and drive housing vibrations from the hull. The lower end of the drive housing is covered by a ride plate which in turn is isolated from the hull by an elastomeric seal to keep the channel watertight.
A disadvantage of such an arrangement is that since the power head is still directly mounted to the drive housing, the amount of vibration generated by these two components may still tend to overcome the isolating capabilities of the main resilient grommet. It has been found that the major source of vibration is the power head, rather than the gear train portion of the drive housing, which is a further disadvantage of the conventional arrangement. Another disadvantage of such an arrangement relates to the rather complicated procedure required for assembling the jet propulsion unit at the same time it is mounted to the hull.
Another design factor of jet propulsion units relates to the fact that to proceed in the reverse direction or "back up", the normally rearwardly directed high pressure propulsion stream of water is directed at a depending angle and generally towards the bow of the craft. This stream is also often directed in the general area of the water intake of the propulsion unit. To prevent cavitation and/or starvation of the jet propulsion impeller due to an excessive amount of entrained exhaust gas in the intake flow, care must be taken to avoid the presence of gas near the intake. Since conventional jet propulsion units emit the engine exhaust at or just below the water surface near the jet outlet, when the unit is placed in reverse, the potential is created for exhaust being emitted into the water and pushed forward toward the water intake by the jet stream.
Thus, an object of the present invention is to provide a mounting system for a jet propulsion unit wherein power head and jet pump housing vibrations are effectively isolated and prevented from being transmitted to the hull.
Another object of the present invention is to provide a mounting system for a jet propulsion unit which is readily installed in the hull of the watercraft.
Still another object of the present invention is to provide a mounting system for a jet propulsion unit wherein the exhaust gases are prevented from entering the water intake when the craft is placed in reverse.